Among copolymers of a conjugated diene and an aromatic vinyl compound, block copolymers which are composed of a conjugated diene block mainly composed of a conjugated diene unit and an aromatic vinyl compound block mainly composed of an aromatic vinyl compound unit exhibit strength and elastic properties equal to those in conventional vulcanized rubbers, without being vulcanized, and therefore, they are widely used alone or as a composition with other thermoplastic resin or a softening agent in fields of automotive components, household electrical components, electrical wire coatings, medical components, footwear, miscellaneous goods, and so on. In addition, it is also known that by hydrogenating an unsaturated double bond derived from a conjugated diene, heat resistance, oxidation resistance, weather resistance, ozone resistance, and so on of copolymers can be improved.
These copolymers are commercially available in various shapes, such as a crumb shape (agglomerate of porous fine particles), a pellet shape, a powdered shape, etc. Especially, those in a crumb shape have such a characteristic feature that due to a high specific surface area, absorption of a softening agent, such as an oil, etc., is good, so that on producing a composition, kneading is readily achieved.
Now, a copolymer composed of a conjugated diene and an aromatic vinyl compound can be, for example, produced by a liquid phase anionic polymerization method of using an organic alkali metal compound as a polymerization initiator in the presence of an inert gas and a hydrocarbon solvent that is inert to the polymerization initiator. In addition, hydrogenation of the copolymer can be carried out in the presence of a hydrogen gas, a hydrogenation catalyst, and a solvent that is inert to the hydrogenation reaction.
Since the thus produced copolymer, hydrogenated copolymer, or mixture thereof is obtained in a state where it is uniformly dissolved or suspended in the solvent used for the reaction, it is necessary to obtain the copolymer by removing the solvent from such a copolymer solution. For the purposes of removing the solvent from the copolymer solution with a less energy, increasing an oil absorption rate of the resulting copolymer, and so on, a variety of solvent separation methods have been investigated so far.
As a representative method for obtaining a copolymer in a crumb shape by removing the solvent from the copolymer solution, there is exemplified a steam stripping method disclosed in PTLs 1 to 3 and so on. Specifically, the steam stripping method is a method in which a mixed liquid obtained by bringing the copolymer solution into contact with steam in the inside of a nozzle, or a suspension liquid obtained by mixing the copolymer solution with an aqueous solvent in the inside of a nozzle, is fed into a desolvation tank where hot water resides; the solvent is distilled off together with a water vapor to obtain an aqueous slurry of a crumb; subsequently, the aqueous slurry is passed through a screen to obtain a hydrated crumb; the hydrated crumb is subjected to dehydration in a mechanical compression mode using rolls, a Banbury type dehydrator, a screw extrusion type press dehydrator, or the like; and the resultant is then dried in a mechanical compression mode using a screw extrusion type dryer, a kneader type dryer, an expander type dryer, or the like, or in a non-compression mode using a hot air dryer or the like. If desired, such a crumb can also be pelletized by mixing with other organic compound or inorganic compound, followed by treatment by using an extruder.
As a representative method for obtaining a copolymer in a pellet shape by removing the solvent from the copolymer solution, there is exemplified a direct devolatilization method for pellet production in which a highly concentrated liquid is charged in a multi-vent extruder, as disclosed in PTLs 4 to 10 and so on. Specifically, the direct devolatilization method for pellet production is a method in which a highly concentrated liquid obtained by subjecting a non-copolymer solution heated to 200 to 300° C. under pressure to flash concentration is mixed with water, if desired and charged in a mixing apparatus having an internal movable mixing section, such as a multi-vent extruder, etc., and a pellet is obtained while allowing the removal of the solvent to proceed.
As a representative method for obtaining a copolymer in a powdered shape by removing the solvent from the copolymer solution, there is exemplified a direct devolatilization method for powder production, as disclosed in PTL 11 and so on. PTL 11 discloses a method in which a mixture prepared by mixing the copolymer solution with a heating fluid, such as water, etc., by a static mixer is heated at 60 to 250° C. and then sprayed from a nozzle to obtain a powdered copolymer, and subsequently, the heating fluid component is removed by using an extruder.